Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Coordinated Multi-Point (CoMP) solution is currently being considered for UpLink (UL) of both HSPA and LTE systems. The main idea of UL CoMP is to pool several sector antennas between and/or within base station sites and process the received multi-antenna signals in a centrally located processing unit. Signals that were previously treated as inter-cell interference are now treated as desired signals. Using a number of antennas, the detection of the multiple user signals can be done either in a linear fashion, e.g., minimum-mean-squared-error (MMSE) equalization approaches, or in a non-linear fashion, e.g., successive interference cancellation (SIC) or maximum likelihood (ML) type approaches. Reconfiguring a traditional cellular system in this way offers the potential to increase both cell-edge bit rates and system capacity.
FIG. 1 illustrates an example of uplink CoMP. On the left of FIG. 1, a conventional system is shown consisting of 2 cells with one active UE in each. The UEs are shown located near the cell edges such that each one creates dominant inter-cell interference in the respective non-serving cells. For example, in cell-1, UE 1 is detected in the presence of inter-cell interference from UE 2. The detection utilizes only the two antennas available at the base station in cell-1. In contrast, in the CoMP system as shown on the right of FIG. 1, the signals from geographically separated antenna sites are transported to a common location for processing within the CoMP cluster. In effect, this creates a 4-antenna base station. UE 1 and UE 2 are then treated as desired signals and detected using a suitable multi-user detection (MUD) approach, either linear or non-linear. Due to the increased number of antennas and MUD, significantly improved performance may be expected. The largest improvement occurs when the signals from UE 1 and UE 2 are received at both sites with approximately equal strength. This occurs most probably for UEs located at the edges of the cells defined in the conventional system. This is why CoMP is viewed primarily as a technique to boost cell-edge throughput.
FIG. 2 illustrates an example CoMP cluster layout highlighting 2 CoMP clusters. Each CoMP cluster is formed from 6 cells (sectors) of a conventional hexagonal layout. At each 3-sector base station site, the arrows indicate sector antennas and the direction in which the antenna main lobes “point.” Note that each sector antenna consists of two physical antennas; hence a total of 12 antennas from two different sites are associated with each CoMP cluster.
The sector antennas associated with each CoMP cluster are connected to the Node B via fiber optic cable, or other transport technologies like micro wave, etc. This can be achieved using a Main-Remote type architecture. Using this kind of architecture, each sector antenna is first connected to a remote radio unit (RRU) which implements RF functionality followed by analog to digital conversion. 6 such RRUs (with two physical antennas each) are then connected to one main unit (MU) which implements all of the standard Node B functionality. The fiber optic cables connecting the RRUs to an MU carry I-Q samples for processing at baseband. In this way, the Node B can jointly process I-Q samples from up to 12 antennas associated with each CoMP cluster.
Hereunder, in the present disclosure, the term “sector” and the term “cell” might be used synonymously herein unless being explicitly indicated.
The UE access procedure for WCDMA is described in 3GPP TS 25.214. The procedures of DCH in Cell_FACH and EUL in Cell_FACH procedure are illustrated in FIG. 3 and FIG. 4 respectively. Tp-p is time duration between the start of the first and the start of the next PRACH preamble if the first PRACH preamble is not acknowledged by demodulating the information over AICH. Tp-m is the time duration between the start of the acknowledged PRACH preamble and the start of the DPCCH preamble. The PRACH preamble ramping procedures in these two cases are similar. In case of the random access procedure for radio link establishment, the coming data includes that for the RRC signaling. In case of the random access procedure for a UE in Cell_FACH, the coming data is usually the traffic data.
When a UE in Cell_FACH has uplink data to transmit, the UE should initiate the random access procedure and start the data transmission after the PRACH preamble is acknowledged by the network. The access delay can be estimated by x*Tp-p+Tp-m, where x is the number of PRACH preamble transmissions that the UE has performed until the UE successfully accesses the network. The primary serving cell knows that there will be a data transmission when it detects a preamble sent by this UE and decides to admit the UE. The primary serving cell has a time of at least Tp-m to prepare the hardware to receive the data from the UE. Tp-m can be at least 2 ms. In practice, there can also be several DPCCH preamble slots before the uplink data transmission takes place, which means the primary serving cell has even more than 2 ms for the hardware preparation to receive the uplink data from this UE.
Hereinafter, the primary serving cell for a UE refers to the cell to which the PRACH preamble selected by the UE during the random access procedure belongs.
It is well-known that UL CoMP has very good gain for short data transmissions compared to non-CoMP in a WCDMA system where soft and softer handover is supported in uplink. This is because the non-CoMP system, which relies on softer handover, may not have enough time to select, configure and activate non-serving cells into the active set for achieving gains for the short data burst transmission (for instance in Cell_FACH) or the signaling transmission procedure for initial radio link establishment. In contrast, in the CoMP system the links to several CoMP cells are available on a faster basis, which means they can be added based on relatively quick uplink measurements (path searcher updates) rather than active set updates. It is further valuable to achieve the CoMP gain for a UE during radio link establishment (RACH procedure) and short data transmission. To achieve the gain in such scenarios, it is required to determine the CoMP cells as quick as possible with a certain acceptable complexity.
The measurement report from UE is usually utilized to determine the secondary CoMP cells. Since such measurement reports are only available after the radio link setup and of low transmitting frequency, it is difficult to achieve the CoMP gain for the signaling and data transmission during radio link establishment stage and the short data transmission.